Communication method and apparatus with transmission of a second signal during absence of a first one

ABSTRACT

A network station receives a principal signal and data. When the principal signal is present or contains information it is transmitted to a receiving station through a communications channel. When the principal signal is absent or does not have a significant information content, the network station transmits the data through the same communications channel in a format such that the data is received and output by a further receiving station.

TECHNICAL FIELD

The present invention relates to a communication method and apparatus,and particularly but not exclusively to a method and apparatus forproviding data communication over a radio frequency channel in additionto voice, image or other data communication.

BACKGROUND ART

In a known radio frequency communication system, for example theINMARSAT-A (TM) Satellite Communication System, as described for examplein “Satellite Communications: Principles and Applications”, Calcutt andTetley, 1st edition 1994, users are connected to a public servicetelephone network (PSTN) through a satellite link to provide voice,facsimile and circuit switched data communication services. Attemptshave been made to add a packet switched data service, in which usersshare a single channel, to the existing services available throughINMARSAT-A. However, this requires the allocation of additional trafficchannels, which adds to the cost of the system.

The document GB-A-2 232 562 describes a digital mobile telephone systemwith a data communications function. If a data communication isasymmetric, time slots which are not needed for communication in onedirection are reassigned for data transmission in the oppositedirection. The time slots may be reassigned to a different dataterminal. However, such a system requires reallocation of part of aphysical channel assigned for communication in one direction so as tochange the direction and/or data terminal to which that part isassigned.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided acommunication system in which a first station is arranged selectively totransmit a principal signal and a data signal. The first stationestablishes a communication channel to a second station and begins totransmit the principal signal to the second earth station. However,during a period in which no information can be transmitted to the secondstation, the first station transmits the data signal in such a way thatit may be decoded by a third station and may also be received by thesecond station without affecting the information decoded by the secondstation.

During voice communication, a proportion of the time during which achannel is kept open is unused, for example when a user is listeningwithout talking, so that the user's transmit channel carries only noise.Likewise, in a facsimile communication, a terminal which receivesfacsimile data only transmits during the handshaking phases of thecommunication; at other times its transmit channel is unused.

In embodiments of the invention the available bandwidth of thecommunication channel is used with greater efficiency, and a datacommunication service may be provided concurrently with another service.

Preferably, the principal signal comprises a voice or facsimile signal.

Where the principle signal is a voice signal, the data signal istransmitted during periods of silence. The first earth station maytransmit a silence code which is decoded by the second earth station toreproduce silence or low-level noise during a voice communication.Preferably, the third earth station interprets the silence code as asignal to receive data.

Alternatively, the first earth station may transmit a signal which isreproduced by the second earth station as acceptable (e.g. low-level)noise or silence but which is decoded by the third earth station asdata, during the periods of silence.

Where the principal signal is a facsimile signal, the first station maydetect whether the second earth station is transmitting facsimile dataand therefore does not need to receive any facsimile data. The firststation transmits the signal including the data signal in such a waythat it does not activate the facsimile terminal at the second earthstation, but activates the third earth station to receive data.

By the above measures, the second and third stations may receive dataconcurrently, with the second earth station only decoding the principalsignal and the third earth station only decoding the data, but withoutinterference between the signals. This aspect of the present inventionextends in particular to the first station, the third station, apparatuswithin the first or third station which implement the essential featuresof the present invention and any method performed by such apparatus.

According to another aspect of the present invention, there is provideda method of encoding data not derived from a voice signal such that itappears to relate to a voice signal to a voice decoder but can bedecoded to reproduce the data by a data decoder, and apparatus forperforming said method.

According to another aspect of the present invention, there is provideda method in which a duplex channel assigned to a facsimile communicationis shared so that, when a facsimile terminal is transmitting facsimiledata in one direction, data is transmitted in the other direction fordecoding by a data decoder in such a way that the facsimile terminal isnot interrupted by the data.

BRIEF DESCRIPTION OF DRAWINGS

Specific embodiments of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a diagram showing the logical connection between earthstations in embodiments of the present invention;

FIG. 2 is a functional block diagram of a land earth station accordingto a first embodiment;

FIG. 3 is a block diagram of a voice codec according to the firstembodiment;

FIG. 4 is a flowchart of the operation of the land earth station;

FIG. 5 is a functional block diagram of a second mobile earth station;

FIG. 6 is a flowchart of the operation of the second mobile earthstation;

FIG. 7 shows the format of a packet transmitted by the land earthstation according to the first embodiment;

FIG. 8 is a functional block diagram of a packet data interface unitaccording to a second embodiment;

FIG. 9 is a block diagram of the packet data interface unit including aPCM codec;

FIG. 10 is a schematic diagram of an APC coder and decoder;

FIG. 11 is a functional block diagram of a land earth station accordingto a third embodiment;

FIG. 12 is a flowchart of the operation of the land earth stationaccording to the third embodiment;

FIG. 13 is a diagram of the packet format of a signal transmitted by theland earth station according to the third embodiment of the presentinvention.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1 shows schematically a communication system in which a networkstation, such as a land earth station (LES) 4 is connected to a publicservice telephone network (PSTN) 2. The LES 4 is arranged to provide asatellite link between a number of users connected to the PSTN 2 and anumber of user terminals, such as mobile earth stations (MES) which areable to receive and decode signals S transmitted by the LES 4 via asatellite, and to encode and transmit signals to the LES 4 via thesatellite. FIG. 1 shows a first MES 6 and a second MES 8, both incommunication with the LES 4 by means of a single channel C.

The nature of the channel C depends on the addressing technique used.For example, in a single channel per carrier (SCPC) system, a channelcorresponds to a single carrier frequency in a half duplex system or apair of carrier frequencies in a full duplex system. In a TDMA system, achannel corresponds to a predetermined time slot in a repeating timeframe of a transmission at a particular frequency or set of frequencies.In a CDMA system, a channel corresponds to a transmission encoded with apredetermined code. The present invention does not depend on theaddressing system used.

The LES 4 also receives data D, addressed to the second MES 8, from thePSTN 2. The data D is stored at the LES 4 until it can be transmitted.The data D may alternatively have been entered directly at the LES 4 orpreviously received from an MES.

In order to establish a communications link with the first MES 6, theLES 4 transmits a predetermined calling code identifying the first MES 6on an LES signalling channel. The calling code includes a service-typecode which identifies whether a voice, facsimile or data transmission isto follow. The first MES 6 acknowledges the calling code on an MESsignalling channel if it is ready to communicate. A communications linkis then allocated by an independent network control station.

If the LES 4 receives data D addressed to the second MES 8, it transmitsa predetermined data addressing code on the LES signalling channel andthe second MES 8 acknowledges the data addressing code if it is ready toreceive the data. The data addressing code includes information on thecommunication link which will be used for data transmission.

A first embodiment of the present invention will be described withreference to FIGS. 1 to 7. In this embodiment, a packet switched dataservice is provided in addition to a voice communication service such asthe INMARSAT-M (TM) system as also described in the Calcutt and Tetleyreference above.

First, a call is set up between the LES 4 and the first MES 6 (step102). The call may be initiated by either the LES 4 or the first MES 6.

During the call, a voice signal V is received from the PSTN 2 by a codec10 of the LES 4. Details of the codec 10 used in the INMARSAT-M (TM)system are described for example in the Calcutt and Tetley reference.This codec 10 uses an Improved Multi-Band Excitation (IMBE) algorithm,shown schematically in FIG. 3. The voice signal V is analysed by a pitchestimation stage 46 which estimates the fundamental frequency {overscore(ω)}_(o) of the voice signal V. A voiced/unvoiced determination state 48evaluates the voicing measure of each of a number of non-overlappingfrequency bands of the voice spectrum and generates a set of decisions{circumflex over (V)}_(k) as to whether the voice signal V is voiced orunvoiced in each band. A spectral amplitude estimation stage 50determines the spectral envelope {circumflex over (M)}₁ of eachfrequency band according to whether the band is determined to be voicedor unvoiced. The values {overscore (ω)}_(c), {circumflex over (V)}_(k),{circumflex over (M)}₁ are encoded for each 20 ms frame of speech toform a series of vectors U₀ to U₇. Error correction information is addedprior to radio frequency modulation and transmission.

However, if the codec 10 does not detect any vocal characteristics inthe voice signal V, it sets the first six bits of vector U₀ to 110010(decimal 50) to form a silence code.

In the first MES 6, another codec 52 decodes the values of {overscore(ω)}_(o), {circumflex over (V)}_(k) and {circumflex over (M)}₁ andsynthesises unvoiced and voiced speech signals at unvoiced speechsynthesizer 54 and voiced speech synthesizer 56 respectively. Theoutputs of the synthesizers 54 and 56 are added to produce a synthesisedspeed signal V_(S). Alternatively, if the codec 52 receives the silencecode, it generates low level “comfort” noise and does not decode anyother information.

The LES 4 includes a packet data interface unit 12 which normallyreceives encoded data from the codec 10 and sends the encoded data (step106) to a radio frequency (RF) modulator 14 for transmission by anantenna 16 to a satellite (not shown). The first MES 6 includes ademodulator for demodulating the signal received from the LES 4.

However, when the packet data interface unit 12 receives a silence codefrom the codec 10 (step 104), it outputs the data D (step 108) in a datapacket 20 as shown in FIG. 7. The data packet 20 includes a silence codeSC, an identity code ID which identifies the second MES 8 to which thedata is to be sent, and the data D itself. The data packet 20 istransmitted via the satellite and is receives by both the first MES 6and by the second MES 8. The process repeats until the call is ended(step 110).

The first MES 6 is set up to receive a voice communication and thereforeresponds to the silence code SC by ignoring the rest of the data packet20. Low level white noise (or other “comfort noise”) is generated by thecodec 52 at the first MES 6 in response to the silence code SC toreassure the user that the communications channel is still open.

In the second MES 8, as shown in FIG. 5 and in the flowchart of FIG. 6,the data packet 20 is received by an antenna 21, RF demodulated by an RFdemodulator 23 and decoded by a decoder 22 which detects whether data isbeing sent (step 112), by detecting the presence of the silence code SC.The decoder 22 extracts the identifying code ID and compares it with acode stored in a comparator 25 (step 114). If the codes match, thecomparator 25 closes a data switch 26 and the decoder 22 outputs thedata D (step 116). The data D is output to a local network or storagedevice, such as an e-mail system. The process continues until the lastblock of data D is received (step 118).

A second embodiment will now be described with reference to FIGS. 4 to 6and 8 to 10. This embodiment is applicable to a system which uses acodec 24 that does not provide silence detection, such as for examplethe INMARSAT-B (TM) system.

The LES 4 in this embodiment includes a packet data interface unit 12having a voice activity detector (VAD) 28 which detects whether theoutput of the codec 24 represents only background noise (step 104). Anexample of this type of detector, (suitable for an LPC codec used in theGSM system) is described in U.S. Pat. No. 5,276,765. The VAD 28activates a switch 30 which switches the output of the packet datainterface unit 12 from the output of the codec 24 (step 106) to thesignal D (step 108) if no voice is detected.

In this embodiment the data D must be encoded so as to be acceptable foracoustic reproduction by the first mobile earth station 6, since thefirst MES 6 is not arranged to recognise a silence code and so cannotignore a received signal as in the first embodiment. The data D istherefore encoded by a data decoder 32 to represent low level noise tothe codec of the first MES 6.

The technique used by the data encoder 32 depends on the type of codecused by the first MES 6. In one example, shown in FIG. 9, the codec 24uses 8-bit pulse code modulation (PCM). The VAD 28 receives the outputof the codec 24 and analyses whether a voice is present. The dataencoder 32 comprises a first functional section 32 a which encodes thedata D into the four least significant bits (LSB) (D_(0 to 3)) inresponse to an output of the VAD 28 indicating that no voice is presentand a second functional section 32 b which outputs zeros as the fourmost significant bits (MSB) in response to the output of the VAD 28, sothat the decoded output signal sounds like low-level noise. The firstand second functional sections 32 a, 32 b allow the output of the codec24 to pass when the VAD 28 outputs a signal indicating that a voice ispresent. Alternatively the data D may be spread-spectrum encoded by thefunctional section 32 a so as to sound like low-level noise. In anotheralternative, shown in FIG. 10, the codec 24 uses adaptive predictivecoding (APC) as in the INMARSAT-B system. In an APC coder, a predictor60 generates an estimation of the input speech signal V in terms of aset of short term reflection coefficients, long term coefficients and anoptimum r.m.s. scaling value. The output of the predictor 60 issubtracted from the actual voice signal by a subtracter 62 and theresidual signal is output to a quantizer 64. The coefficients and thequantized residual signal are transmitted. In an APC decoder 53, thequantized residual signal is sent to an inverse quantizer 66 while thecoefficients are sent to another predictor 70 so as to reconstruct thespeech signal V_(S). Selected coefficients such as the reflectioncoefficients may be used to carry the data, while the remainingcoefficients are set to zero so that no sound is generated at the codec53 of the first MES 6.

Referring to FIGS. 5 and 6, in this embodiment the decoder 22 of thesecond MES 8 recognizes (step 112) an encoded sequence in the dataencoded by the data encoder 32 and compares (step 114) the encodedsequence with a code stored in a comparator 25. If the encoded sequencematches the code, the comparator closes the data switch 26 and data D isoutput (step 116) from the second MES 8 as in the first embodiment.

A third embodiment will now be described with reference to FIGS. 11 to13. In this case, the LES 4 is connected through the PSTN 2 to areceiving facsimile terminal which sends facsimile protocol signal andthe first MES 6 is connected to a transmitting facsimile terminal whichsends both protocol signal and facsimile data. The protocol signals mayfor example comply with CCITT (not ITU-T) recommendations T.30 and T.4.The protocol signals establish the communication mode of the terminalsduring a pre-message procedure, correct errors and indicate multiplepages during message transmission and signal and acknowledge the end ofa transmission during a post-message procedure. Both facsimile terminalsoperate in half-duplex mode, so that they cannot receive data whilstthey are transmitting. However, a full duplex link is set up between theLES 4 and the first MES 6, since the same channel type is used for voiceand facsimile communication.

The LES 4 includes a facsimile interface unit (FIU) 36 which convertsthe facsimile protocol signals F from the PSTN 2 to data suitable fortransmission by the radio frequency interface unit 14, and converts datareceived from the radio frequency interface unit 14 to facsimile signalssuitable for sending through the PSTN 2. Such a facsimile interface unitis described for example in British Patent Publication No. 2286739 orInternational Patent Publication No. WO 92/02100.

A monitor 38 is connected to the FIU 36, so as to detect (step 122) whenthe FIU 36 is receiving or is about to receive facsimile data from thefirst MES 6. This may be done either by detecting the receipt offacsimile data from the first MES 6 or by identifying the end of thepre-message procedure or multi-page procedure by monitoring the protocolsignals sent by the LES 4. For example, the monitor 38 may be activatedby a CFR (confirmation to receive) signal sent by the LES 4.

The monitor 38 then activates a switch 40 which outputs the data D (step124) to the radio frequency interface unit 14, for reception by thesecond MES 8. Successive blocks of data D are sent until the monitor 38detects (step 126) the end of the transmission of the facsimile data bythe first MES 6, for example by detecting an MPS (multipage signal) orEOM (end of message) signal from the first MES 6. If the message is tocontinue (step 128), the monitor 38 detects (step 122) when the firstMES 6 is sending more data and again activates the switch 40.

The data D is encoded by a data encoder 42 arranged according to thetype of communication system. In the INMARSAT-B (TM) system, the LES 4sends an “idle” code to an MES while the MES is transmitting facsimiledata, to indicate that no information is being sent from the LES 4.Therefore, the data encoder 42 for use with INMARSAT-B (TM) encodes thedata in a format shown in FIG. 13, in which a data packet 44 includes anidle code IC, an identifying code ID and the data D. The first MES 6receives this data packet, decodes the idle code IC and ignores the restof the packet. Meanwhile, the second MES 8, as shown in FIG. 5, isactivated when the decoder 22 receives the idle code IC. The decoder 22then decodes the identifying code ID and outputs it to the comparator24. The data switch 26 is activated to output the data D if theidentifying code matches that stored in the comparator 25.

In an alternative system, such as the INMARSAT-M (TM) system, thereceiving portion of the first MES 6 is automatically idle duringfacsimile data transmission, unless it receives a sequence of 72consecutive “ones”. Therefore, the data encoder 42 need only encode thedata so as to avoid reproducing such a sequence. As before, the dataencoder inserts an identifying code ID to identify the second mobileearth station 8 for which the data communication is intended.

The first MES 6 may also be operable to receive data in the same manneras the second MES 8 and the second MES 8 may also be operable to receivevoice, facsimile or other signals in the same manner as the first MES 6.

The data formats shown in FIGS. 7 and 13 do not necessarily correspondto the sequential order in which the data is transmitted. The blocks ofdata may be interleaved, to minimize the effect of short bursts ofnoise.

Although in the above embodiments the first MES 6 and the second MES 8are physically separate, they may alternatively share some functionalblocks and may be integrated in a single earth station. For example, thefirst MES 6 and the second MES 8 may share an aerial but have separateradio frequency demodulators and data decoding sections. The decodedvoice, fax or data signals may be transmitted to the same network orstorage device.

INDUSTRIAL APPLICABILITY

Although the above specific embodiments have been described withspecific reference to the INMARSAT-M (TM) and INMARSAT-B (TM) systems,the present invention is also applicable to other communications systemsin which it is possible for two users to receive a common channel. Forexample, the present invention is applicable to systems using radiolinks, such as satellite communications systems or terrestrial cellularsystems. One such cellular system is a GSM (Special Mobile Group)standard system, which uses Time Division Multiple Access (TDMA) andincorporates silence detection.

The present invention may be advantageously applied to a TDMA system, byselective insertion of data packets in different time slots according towhich channels are silent in each time frame. In this way, excessivedelay in data transmission, caused by some channels being constantlyunavailable, for example because of incessant talking on them, may beavoided. Thus, it can be seen that successive packets of a datatransmission need not occupy the same channel, and may occupy more thanone time slot in the same TDMA time frame.

Furthermore, the invention is not limited to use in voice or facsimilecommunications. For example, the principal signal may be a conditionalreplenishment image signal, in which the amount of image data per framevaries according to the variation between frames and the data signalcould be transmitted with the image data when only a small quantity ofimage data need be sent.

The terms LES and MES are used purely by way of example and theinvention is not limited to fixed, mobile, handheld, land-, sea- orair-based stations or terminals or any particular combination of theaforementioned.

The functional blocks shown in the accompanying drawings do notnecessarily correspond to discrete physical units, but may beimplemented in many ways known in the art.

What is claimed is:
 1. A method of radio-frequency communication toseparate first and second mobile terminals, the method comprising:storing data for transmission to said second mobile terminal; setting upa communication with said first mobile terminal via a radio-frequencychannel; detecting an absence of information in said communication; andtransmitting one or more data packets derived from said data andaddressed to said second mobile terminal in said radio-frequency channelduring said absence.
 2. A method as claimed in claim 1, wherein the oneor more data packets are transmitted with a predetermined codeindicative of said absence of information.
 3. A method as claimed inclaim 1, wherein the one or more data packets include datarepresentative of said absence of information in a format arranged fordecoding by the first mobile terminal.
 4. A method as claimed in claim1, wherein said communication is a voice communication.
 5. A method asclaimed in claim 1, wherein said channel is a duplex channel and thetransmitting step comprises transmitting said one or more data packetswhile the first mobile terminal is in a transmit-only state.
 6. A methodas claimed in claim 5, wherein the transmit-only state of the firstmobile terminal is determined by detecting a signal transmitted by thefirst mobile terminal, said signal being indicative of saidtransmit-only state.
 7. A method as claimed in claim 5, wherein thetransmit-only state of the first mobile terminal is determined bydetecting a command signal transmitted to the first mobile terminal,which commands the mobile terminal to enter said transmit-only state. 8.A method as claimed in claim 1, wherein said communication is afacsimile communication.
 9. A method as claimed in claim 1, wherein saidone or more data packets are transmitted with a predeterminedidentifying code associated with said second mobile terminal.
 10. Amethod as claimed in claim 1, wherein said radio-frequency channel istransmitted via satellite.
 11. A method as claimed in claim 1, whereinsad radio-frequency channel is a terrestrial cellular channel. 12.Apparatus for radio-frequency communication to separate first and secondmobile terminals, comprising: storing means for storing data fortransmission to said second mobile terminal; set-up means for setting upa communication with said first mobile terminal via a radio-frequencychannel, the communication comprising information not addressed to saidsecond mobile terminal; detecting means for detecting an absence ofinformation in said communication; and transmitting means fortransmitting one or more data packets derived from said data andaddressed to said second mobile terminal in said radio-frequency channelduring said absence.
 13. Apparatus as claimed in claim 12, wherein saidtransmitting means is arranged to output said one or more data packetstogether with a predetermined code indicative of said absence ofinformation.
 14. Apparatus as claimed in claim 12, wherein saidtransmitting means is arranged to output said one or more data packetsin a form representative of the absence of information.
 15. Apparatus asclaimed in claim 12, wherein said communication is a voice-typecommunication.
 16. Apparatus as claimed in claim 12, including statedetecting means for detecting a transmit-only state of the first mobileterminal, the transmitting means being operable to output the one ormore data packets in response to detection of the transmit-only state bythe state detecting means.
 17. Apparatus as claimed in claim 16, whereinthe state detecting means is operable to identify said transmit-onlystate by detecting a signal indicative of said transmit-only state,which is transmitted by the first mobile terminal.
 18. Apparatus asclaimed in claim 16, wherein the state detecting means is operable toidentify said transmit-only state by detecting a command signal which istransmitted to the first mobile terminal to command the first mobileterminal to enter said transmit-only state.
 19. Apparatus as claimed inclaim 12, wherein said communication is a facsimile communication. 20.Apparatus as claimed in claim 12, wherein said one or more data packetsinclude a predetermined identifying code associated with said secondmobile terminal.
 21. Apparatus as claimed in claim 12, wherein thechannel is a satellite channel.
 22. Apparatus as claimed in claim 12,wherein the channel is a terrestrial cellular channel.